All of the objectives of this project involve the use of optical methods for monitoring neuron activity. These methods are based on the finding that certain dyes, when bound to membranes, change their optical properties in response to changes in membrane potential. In invertebrate ganglia optical monitoring detects the individual action-potentials in individual cell bodies. Using a 124 element photodiode array positioned in a microscope image plane we have been able to monitor the activity of many neurons of Aplysia or Pleurobranchaea buccal ganglia. We think we are approaching the capability of monitoring most of the spike activity in such preparations. This would provide a powerful tool for determining all of the neurons active during particular behaviors and for testing hypothese about neuronal control of simple behaviours. We are particularly interested in the nuronal basis of simple forms of learning. Additional behavioral experiments and optical measurements in semi-intact preparations are proposed for the immediate future. These would be followed by attempts to determine how the interactions between neurons changed to accomplish the behavior. In vertebrate nervous systems optical monitoring does not yet have single cell resolution. The signals represent average changes in membrane potential of the neurons and processes imaged onto the individual detectors. At present it is possible to measure changes in activity in rat sensory cortex in response to whisker movement or light flashes. We plan experiments to improve the signal-to-noise ratio in these experiments, to develop the use of optical measurements in cat visual cortex, and to determine if there is odor dependent localization of activity in the salamander olfactory bulb. Neurosurgeons could more accurately locate primary epileptogenic areas of cortex if they could identify epileptic foci in the inter-siezure interval. Since neurons in the epileptic foci are said to be somewhat depolarized during the intersiezure interval, voltage sensitive dyes might be used to locate these cells. We plan experiments on model systems to test the feasibility of two possible approaches toward localization.